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Synthetic Metals 160 (2010) 291–296

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

agnetoresistance for organic semiconductors: Small molecule, oligomer,onjugated polymer, and non-conjugated polymer

.L. Martina,∗, J.D. Bergesonb, V.N. Prigodina, A.J. Epsteina,c

Department of Physics, The Ohio State University, Columbus, OH 43210-1117, USANational Renewable Energy Lab, Golden, CO 80401-3305, USADepartment of Chemistry, The Ohio State University, Columbus, OH 43210-1173, USA

r t i c l e i n f o

rticle history:eceived 6 June 2009eceived in revised form 6 January 2010ccepted 7 January 2010vailable online 27 January 2010

a b s t r a c t

Organic semiconductor (OSC) devices have been shown to have a large magnetoresistance (MR) responseat room temperature for relatively small-applied magnetic fields of 0.1–100 milli-Tesla (mT). This largeMR is not limited to one class of organics, but is seen in small molecules, oligomers, conjugated polymers,and non-conjugated polymers. In this paper, data is presented on the MR effect for the poly(phenylenevinylene) (PPV) derivative “Super Yellow,” for poly(vinylenecarbazole) (PVK), for alpha-sexithiophene

eywords:rganic semiconductoragnetoresistance

ecombinationLED

(�-6T), and for tris(8-hydroxyquinoline) aluminum (Alq3). The data is analyzed in the context of theMagnetoresistance by the Interconversion of Singlets and Triplets (MIST) model. The MR data of Alq3 formagnetic fields of less than 1 mT are fitted to a polynomial expansion, and an estimate for the hyperfineinteraction constant, which is consistent with values for small molecules, is extracted from the fittingparameters. Curve fits at fields in the 100 mT range are also presented and they show that there exist twokinds of magnetic field behavior, inverse square root, and inverse even orders. Furthermore, the scaling

orde

factor at this range is one

. Introduction

The first report of a magnetic field influencing the electricalroperties of an organic semiconductor was by Frankevich andalabanov for single crystal anthracene [1,2]. They observed a% change in the photocurrent with applied magnetic field, upo 100 mT. In 2003, Kalinowski et al. reported a change of 2%n the resistance of non-crystalline Alq3 for applied magneticelds of up to 0.5 T [3]. This anomalously large magnetoresis-ance, which occurs at room temperature for magnetic fields asmall as 50 mT, has also been observed in the conjugated polymersolyflourene [4] and poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-henylene vinylene] (MEH-PPV) [5] and in the oligomer �-6T [6]. Inhis paper, we present the MR data for the conjugated polymer PPVerivative Super Yellow, the non-conjugated polymer PVK, and themall molecule Alq3. The study of magnetoresistance for organicemiconductors has sparked much interest [6–12], but due to the

on-intuitive dependence of the behavior upon extrinsic parame-ers such as active film thickness, temperature, and applied voltage,ttempts at a full quantitative model of the fundamental mecha-isms at work are still in progress. Three models have come to the

∗ Corresponding author.E-mail address: jessem@mps.ohio-state.edu (J.L. Martin).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.01.009

r of magnitude larger than that found in the 3 mT range.© 2010 Elsevier B.V. All rights reserved.

forefront in the discussion of organic magnetoresistance. These arethe MIST model [7,13], the two-site bipolaron model [14], and theexciton–polaron model [9]. In this paper, MR data are presented fora variety of organic semiconductors and it is shown that the data isconsistent with the MIST model.

2. Device fabrication

Devices are in OLED configurations and consist of an etched andcleaned ITO substrate with a spin coated and annealed PEDOT:PSSfilm acting as the hole injecting layer. A film of the organic semicon-ductor is thermally sublimed in vacuum at a pressure of 2 �Torr, orfor polymers in solution, spincoated and annealed. A 10 nm layer ofCa is followed by a capping layer of Al, and forms the opposite elec-trode interface. Contact is made directly to the ITO and the Al. Allcleaning and device fabrication steps are conducted in a class 1000clean room, and all fabrication steps are conducted in inert atmo-sphere glove boxes with oxygen and moisture levels held below1 ppm. All organics are purified in house by means of gradient sub-limation, and stored in inert atmospheres so as to avoid accidental

doping from oxygen and moisture. After completion, the samplesare stored in a freezer under inert atmosphere, and never allowedcontact with ambient atmosphere.

Samples are transferred in sealed containers from the glove boxto either a Quantum Design physical properties measurement sys-

2 tic Metals 160 (2010) 291–296

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Fig. 1. shows the change in resistance as a function of applied magnetic field and

92 J.L. Martin et al. / Synthe

em (PPMS) with a superconducting magnet for high magnetic fieldtudies, or to a resistive electro-magnetic coil for low magnetic fieldtudies. All electrical characterizations are done with a Keithley400 source measurement unit.

. Results

Fig. 1 shows results for a device utilizing the PPV derivativeSuper Yellow” as the active layer with a thickness of 100 nm. Theagnitude of the MR percentage change decreases with increasing

oltage in Fig. 1a, while in Fig. 1b and c, the MR percentage changencreases with increasing voltage. Fig. 1d displays a non-monotonicehavior as a function of voltage. The Super Yellow device dis-lays an oscillatory behavior in the MR magnitude as a functionf temperature.

Fig. 2 shows data collected from a PVK device with a thickness of00 nm. Fig. 2a, c and d shows increasing MR signals as the appliedoltage is increased, but Fig. 2b has a non-monotonic behavior.he maximum MR signal in these figures seems independent ofemperature.

Figs. 3 and 4 show data for Alq3 devices with various thicknessest various temperatures and for various hole injecting layers. Fig. 3aresents data taken from a 200 nm thick sample at 200 K. As theoltage is increased, the signal decreases. In Fig. 3b, the active layers 100 nm thick and again the data is taken at 200 K. The data forhis device decreases as the voltage increases, and then begins toncrease with increasing voltage. Fig. 3c is for the same 200 nm thickevice in Fig. 3a, but at 100 K. It now has a non-monotonic behavior,ut starts by increasing, then decreasing, in contrast to Fig. 3b.

Fig. 4a is data taken from a 200 nm thick device at three differ-nt temperatures, and voltages chosen so as to maintain the sameurrent density in each sweep. As the temperature decreases, theagnitude of the MR signal decreases. Fig. 4b is for a device consist-

ng of Au, which replaces the ITO and PEDOT:PSS, 200 nm of Alq3,nd Ca capped with Al. The data is taken at 300 K. As the voltagencreases, the MR decreases.

. Discussion

When a voltage is applied to a low mobility semiconductor, apace charge will develop if the charge is injected faster than it canove away from the electrodes. The solution for the current density

,

= (3/2)ε0εr[��e�h(�e + �h)/2�r]1/2V2/L3 (1)

as found analytically by Parmenter and Ruppel [15]. In this solu-ion, L is the thickness of the semiconductor film, V is the voltagecross that film, ε0 is the dielectric constant of space, εr is theelative dielectric constant of the semiconductor, and �h(e) is theobility of the holes (electrons) in the semiconductor. The term �r

s the e–h recombination mobility,

r = ε0εrˇ/2e (2)

here ˇ is the recombination coefficient [13]. If �r decreases, theurrent density increases because the voltage due to the spaceharge is alleviated as the charge carriers intermingle.

The magnetic field is capable of influencing the injected currenty means of influencing the recombination of the system. Triplettates that are capable of converting into singlet states will havedecreased lifetime. The hyperfine interaction will allow triplets

o convert into singlets if the states degenerate in energy. Applica-ion of a magnetic field will lift this degeneracy by means of Zeemanplitting, thus hindering the conversion and lengthening the tripletifetime. This decreases the recombination of the e–h pairs in theystem. However, the organic semiconductor is not capable of sus-

voltage for a PPV (Super Yellow) device at various temperatures. The thickness ofthe PPV layer is 100 nm. The maximum MR fluctuates from 3% to 5% to 3% and backto 5% as temperature is increased. Insets show an MR cross-section at 200 mT and−200 mT.

J.L. Martin et al. / Synthetic Metals 160 (2010) 291–296 293

Fig. 2. shows the change in MR percentage as a function of applied magnetic fieldand voltage for PVK devices at various temperatures. The thickness of the PVK layeris 100 nm. The data show a weak dependence upon temperature. Insets show an MRcross-section at 200 mT and −200 mT.

Fig. 3. shows MR percentage change for Alq3 devices at various temperatures andthicknesses for a variety of applied voltages. (a) Has an active layer thickness of

200 nm and the data was taken at a temperature of 200 K. (b) Has an active layerthickness of 100 nm and the data was taken at a temperature of 200 K. (c) Has anactive layer of 200 nm and the data was taken at a temperature of 100 K. Insets showan MR cross-section at 200 mT and −200 mT.

taining an unlimited free charge density. If the charge density is ableto saturate, the system crosses over into a density limited regimefor which decreased recombination leads to decreased current [6].In this density limited regime the current J has a behavior described

by,

J = eˇLc2(V) (3)

where c(V) is a function dependent upon the applied voltage V [13].

294 J.L. Martin et al. / Synthetic Metals 160 (2010) 291–296

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Fig. 5. A schematically illustrates the dependence of the current density, J, on therecombination mobility, �r. �c is the critical value at which J reaches a maximum,and �inf is the value at which the inflection point occurs in the graph of J. For valuesof �r less than �c , J follows Eq. (3) and has a linear dependence upon �r . For �r > �c ,J follows Eq. (1) and has a �r

−1/2 dependence. (b) Illustrates the behavior of �c (and

dependencies. Given the current data, it is impossible to make ageneral statement about the temperature dependence. It shouldalso be noted that while the voltage ranges for the various temper-atures may appear arbitrary, they are chosen to display the initialonset of the MR, and limited to not exceed certain current densi-

ig. 4. shows the percentage change in MR for Alq3 devices for different hole inject-ng electrodes. (a) Is for data that utilizes PEDOT:PSS as the hole injecting layer. (b) Isor data that utilizes gold for the hole injecting layer. All data shown for this deviceas taken at 300 K. The inset shows an MR cross-section at 200 mT and −200 mT.

Considering the space charge limited regime with the densityimited regime, it can be shown that the current density behaves innon-monotonic way as a function of �r as shown in Fig. 5a, with

he critical recombination mobility, �c, defined by,

c = (9�/32)1/3(�e + �h)(2 + �e/�h + �h/�e)−1/3(V/Vc)4/3 (4a)

c = e/(ε0εr)c(V)L2 (4b)

For the case of the recombination mobility at zero applied mag-etic field, �r(0), less than �c, the MR will be positive. If �r(0) isreater than �c, then the MR will be negative. MR inversion occurshen �c passes through �r(0) [6]. If �r(0) is greater than �c, theR response will grow as �r(0) and the inflection point �inf draw

loser, as this is the point of maximum slope, and the MR willecrease as �r(0) and �inf separate. As temperature, applied volt-ge, and device thickness are varied, �c (and subsequently �inf)ill change in value as shown schematically in Fig. 5b, c, and d.

his accounts for the monotonic behaviors seen in Figs. 1a, 2b,nd 3a, which decrease in magnitude as V increases, and Figs. 1bnd c and 2a, c and d, which increase in magnitude as V increases.here are two possible causes of non-monotonic behavior in theR signal. The first is for �inf to pass through �r(0) as the rele-

ant parameters are varied. This will lead to an increase, followedy a decrease in the MR signal. The second cause is for V to pass

hrough Vc. This can be either an increase followed by a decreaser a decrease followed by an increase, depending upon whether orot �inf is approaching �r(0). Either of these causes could lead tohe behavior observed in Figs. 1d and 3c. However, consider thepecific example of �c < �r(0) < �inf and V < Vc. As V approaches Vc,

consequently �inf) as a function of thickness, d, while the temperature dependenceis shown in (c). The upper curve in (c) is for the case when Vbi + V is within the gap ofthe organic semiconductor and the lower curve is for when Vbi + V exceeds the gap.In (d), Vc is the critical voltage at which �c reaches maximum value.

�inf will move away from �r(0) as it increases in value, leading toa decrease in MR. As V passes through Vc and surpasses it, �inf willbegin to approach �r(0), leading to an increase in MR. This is exactlythe case as seen in Fig. 3b.

It has been shown that dI/I and dEL/EL can increase together [16],and while this result may seem unexpected given that the recom-bination rate is decreasing, this result agrees with the MIST model.As the recombination rate for individual charge pairs decreases,the number of recombination events increases due to an increasedcurrent density. This increase in charge carriers and recombinationevents leads to the increase of dI/I and dEL/EL.

The temperature dependence is the most difficult aspect tointerpret as not only �c depends upon temperature, but in prin-ciple so can �r(0). As both these quantities change, combinationsof parameters can lead to apparently strong or weak temperature

Fig. 6. Displays a curve fit to the data for an Alq3 device of the form d1B2 + d2B4, withd1 = −0.01085 mT−2 and d2 = 0.00062 mT−4 from −1 mT to 1 mT.

J.L. Martin et al. / Synthetic Metals 160 (2010) 291–296 295

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ig. 7. Displays data from 100 mT to 250 mT range for PVK and Alq3, respectivelyith curve fits of the form f1/B2 + f2/B4.

ies and power throughputs. If the voltage is too high it can causeremature burn out, and/or lead to electrochemical reactions thathange the very nature of the OSC. If the voltage is too low, it may beelow the onset voltage. These devices display a diode type IV curveehavior, with current below the onset voltage corresponding to

ntrinsic carriers, and current above the onset voltage correspond-ng to injected carriers. No MR response is observed below the onsetoltage, which agrees with literature [16] and the MIST model. Ashe temperature is varied, the onset voltage also changes [17] andhe experiment is shifted to different voltage regimes.

At low magnetic fields, the magnetoresistance is a result of com-etition between Zeeman splitting from the applied field B, andhe randomly oriented internal hyperfine field Bhf. As a result, theimensionless MR is a function of the dimensionless parameter= B/Bhf. In the limit x → 0, the expansion of the MR as a functionf x takes the form b1x2 + b2x4. . . This originates from perturbationheory as the externally applied magnetic field is a perturbation tohe hyperfine field. The expansion reflects the fact that the MR is an

ven function of B with no dependence upon the orientation of theeld vector, and that the MR goes to zero as B goes to zero, by defini-ion. In the case of x → ∞, the expansion of the MR is c1/x2 + c2/x4. . .he second expansion is a result of the hyperfine field being a per-

Fig. 8. (a and b) Data from the 100 mT to 500 mT range for �-6T and from the rangeof 100 mT to 250 mT for PPV with curve fits of the form f1/B2 + f2/B4 (solid) and g1/

√B

(dashed). The inverse root fit is a better empirical fit.

turbation to the applied field. The coefficients for these expansionsare of order 1.

Fig. 6 shows a curve fit for an Alq3 device of the form d1B2 + d2B4

with Bhf contained in d1 and d2 in the range of 1 mT. It is found thatd1 = −0.01085 mT−2 and d2 = 0.00062 mT−4. It is noted that the MRsignal is independent of applied magnetic field orientation, so thedata should depend only upon even orders of B. As can be seen,a fit to the data of the form d1B2 + d2B4 describes the curve quitewell (Fig. 6). Taking

√−d1/d2 provides an estimate for Bhf by can-celling the other constants. The result Bhf ≈ 4.1 mT is obtained byusing these fitting parameters, which is in good agreement withestimates for organic materials [18].

At larger fields, i.e., B » Bhf, the data should follow an expansionof inverse even powers. Fig. 7 shows a fit of the form f1/B2 + f2/B4

for PVK and Alq3. Both fits describe the data quite well, but if Bhf isextracted from the coefficients in the same manner as before by tak-ing

√f2/f1, we find Bhf to be roughly 10 times larger, ranging from

10 mT to 90 mT. Furthermore, PPV and �-6T are better describedby g1/

√B, as shown in Fig. 8. It has been shown that at magnetic

fields of order 1–10 T, the magnetoresistance behaves differentlythan at small fields of 1–10 mT [19,20]. This difference of behavior

is attributed to a degeneracy that is created in closely bound e–hpairs as the magnetic field increases and is dependent upon thepopulation distribution of e–h pairs as a function of charge sepa-ration [20]. Both PPV and �-6T have long conjugated paths as PPV

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s a conjugated polymer, and �-6T is a conjugated oligomer. PVK,n the other hand is a non-conjugated polymer, and Alq3 is a smallolecule. While it is expected that all of these materials would

ave different population distributions of e–h pairs as a functionf separation, it is also expected that they would fall into two cat-gories of behavior based upon the similarities and differences ofheir chemical structure.

. Conclusion

Data has been presented for PPV (Super Yellow), PVK, �-6T, andlq3 under various operational parameters. The data were analyzedsing the MIST model, and were determined to support the modeluite well. An analysis of the behavior at low field (1–10 mT) andt high field (100 mT) has been presented, along with a contrastn the behaviors of molecules with paths of conjugation versus

olecules with localized conjugation circuits. The MIST modelrovides insight into the nature of organic MR and a means ofxplaining the different types of behavior. A potential applicationf this model is in the field of photovoltaics because the magneticeld influences the recombination of e–h pairs within the system21,22].

cknowledgements

This work was supported in part by DOE Grant No. DE-FG02-1ER45931, NSF Grant No. DMR-0805220, and OSU Institute foraterials Research. We thank Covion Organic SemiconductorsmbH, presently Merck OLED Materials GmbH, for the supply of

Super Yellow.”

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tals 160 (2010) 291–296

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